Nearly all conventional boilers generally in practical use are air combustion boilers which are disclosed, for example, in Patent Literatures 1 and 2.
The air combustion boilers have a problem of lowered heat efficiency because of sensible heat loss owing to a nitrogen component in the air. Thus, oxygen-enriched combustion has been studied as a measure for enhanced heat efficiency in the combustion. The oxygen-enriched combustion, which has a nitrogen component reduced relative to that in the air combustion, is known to have lowered sensible heat loss and thus enhanced heat efficiency.
Meanwhile, in a boiler or the like, pure oxyfuel combustion and a combination thereof with flue gas recirculation have been recently suggested (see, for example, Patent Literature 3). Attention has been paid to such oxyfuel combustion as an effective way since, if employed, nearly all flue gas may be changed into carbon dioxide (CO2) which may be captured using a simplified separator.